In a Passive Optical Network (PON) which relies on including a guard band between upstream transmissions from each outstation, the size of the guard band is related to the differential distance between the outstations. Assuming no measurement of the relative geographic distances from the head end to each of the outstations is made, the guard band increases according to the differential distance at around 10 microseconds per kilometer distance. For example, on a PON covering a 10 km distance with 32 outstations, 3200 microseconds must be allowed for guard band when interrogating all outstations. To maintain an upstream efficiency of 80%, the total active transmission time for all outstations then amounts to 12800 (4×3200) microseconds and the total time to interrogate all outstations is 16 milliseconds.
One problem arising from this is that many telecommunications services require a bounded transmission delay, typically no more than a few milliseconds for any given network element. A delay contribution of 16 milliseconds is therefore unacceptable. In many cases efficiency falls significantly as the total interrogation time is reduced.
The simplest known form of Gigabit Ethernet PON (GEPON) dispenses with ranging or marshalling of outstations and can nevertheless achieve useful performance within a limited set of parameters. One technique is to limit the differential distance from the splitter to each outstation. For example, the splitter may be placed 9 km from the headend, and the outstations all clustered within 1 km of this splitter. In such an arrangement, a GEPON with 80% upstream bandwidth efficiency for a 16-way split, a 1 km differential distance between outstations and a 4 ms latency can be achieved.
A problem with such an arrangement is that increasing the differential distance or split ratio leads to a reduction in bandwidth efficiency or an unacceptable latency, meaning that such a simple GEPON has limited applicability.
An alternative technique is to use a complex marshalling system, whereby a special messaging protocol is used between the headend and each outstation to measure the timing, and to marshal, or build out, the exact outstation transmit timing in order to reduce the inter-burst guard bands to an acceptable level. Problematically, this marshalling technique requires specialised silicon at both ends of the system. Such a system tends to be proprietary and difficult to standardise because it requires radical changes to long-established Ethernet-MAC designs.
The present invention achieves efficiency close to that of a fully ranged system, but without the need for outstation delay build out.